Semiconducting organic materials are attracting a great deal of interest due to their processability and the broad spectrum of optical and electronic properties that may be selected according to the structure of the organic material.
One application of such materials is in switching devices, in particular as organic field effect transistors as described in, for example, Adv. Mater. 1998, 10(5), 365-377. Another application is in opto-electrical devices using a semiconducting organic material for light emission (an organic light emitting device or “OLED”) or as the active component of a photocell or photodetector (a “photovoltaic” device). The basic structure of these devices is a semiconducting organic layer sandwiched between a cathode for injecting or accepting negative charge carriers (electrons) and an anode for injecting or accepting positive charge carriers (holes) into the organic layer. In an organic electroluminescent device, electrons and holes are injected into a layer of electroluminescent semiconducting material where they combine to generate excitons that undergo radiative decay. Holes are injected from the anode into the highest occupied molecular orbital (HOMO) of the electroluminescent material; electrons are injected from the cathode into the lowest unoccupied molecular orbital (LUMO) of the electroluminescent material. In WO 90/13148 the organic light-emissive material is a polymer, namely poly(p-phenylenevinylene) (“PPV”). Other light emitting polymers known in the art include polyfluorenes and polyphenylenes. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinoline) aluminium (“Alq3”). Light emitting polymers such as polyfluorenes and polyphenylenes are advantageous in that they are solution processable. In particular, solution processable light emitting polymers may be inkjet printed as described in EP 0880303 to produce high information content displays, in particular full colour displays.
An essential requirement for an effective OLED is efficient injection of holes and electrons into the electroluminescent layer of the OLED. Therefore, a focus in the OLED field is the development of hole and/or electron transporting materials to be used in combination with the electroluminescent material. An effective hole transporting material has a HOMO level that lies between the workfunction of the anode and the HOMO level of the electroluminescent material. Furthermore, emission in a typical OLED is usually provided predominantly by the material having the smallest HOMO-LUMO bandgap and so the charge transport material should have a larger HOMO-LUMO bandgap than the electroluminescent material if emission from the charge transport material is undesirable.
Another focus in the OLED field is the development of full colour OLEDs, i.e. OLEDs comprising red, green and blue electroluminescent materials. A drawback of many blue organic electroluminescent materials is that their emission is a relatively pale blue, due to an insufficiently large HOMO-LUMO bandgap, when compared to the standard blue as defined by 1931 CIE co-ordinates.
WO 99/48160 discloses hole transporting co-polymers “TFB” and “PFB”:

However, the above identified co-polymers have drawbacks. In particular, the HOMO levels of these materials are not ideally matched to the workfunction of ITO which negatively affects their hole transporting properties.
WO 03/000773 discloses a copolymer of “TFB-PFB” wherein dioctyifluorene units of the above co-polymers are absent.

Polymers of this type wherein a repeat unit such as fluorene is absent can suffer from insolubility, particularly at higher molecular weights. Repeat units such as fluorene could be incorporated Into a polymer containing TFB and PFB units however this breaks up the regularity of directly linked TFB and PFB units.
JP 09-151371 discloses tri- or tetra-amine hole injecting materials of formula (A)
wherein R1-5 is selected from H, methyl, methoxy, phenyl, trifluoromethyl, hydroxy, hydroxymethyl, formyl, NH2, a double bonded group and an epoxy ring; and n is 1 or 2. When used in a device, this material is provided as a hole injecting layer with a separate emissive layer of Alq3.
Similar compounds are disclosed in WO 96/22273 and JP 11-251068. These materials are deposited by evaporation.
Materials of this type are incorporated into a polymer in WO 97/33193 which discloses polyarylpolyamines such as an acrylate polymer of formula (B):

This material is cured to form a hole transporting layer, however the polymer derived from this material is not soluble due to cross-linking of the acrylate groups and as a result the polymer may only be formed by depositing a layer of the above monomer and curing it. Furthermore, use of acrylate groups for polymerisation does not provide any control over regioregularity of the resultant polymer.
It is therefore an object of the invention to provide a solution processable polymer having improved hole transporting properties without adversely affecting the colour of emission of the polymer by narrowing of the HOMO-LUMO bandgap.